This invention relates to a sewing machine for forming lock stitches by an upper thread and a lower thread, and more particularly to a sewing machine capable of stitching up with the upper thread having a low tension.
The conventional lock stitch sewing machine has been constructed such that an upper thread is clamped by a clamping force of a predetermined value throughout the period of forming a lock stitch by a thread tension device in which two thread tension discs are pressed against each other by a spring and the upper thread is passed through a space between the thread tension discs, so that a tension can be rendered to the upper thread.
In the conventional sewing machine, unless a tension of about 120 g is rendered to the upper thread, a proper lock stitch cannot be formed in the case of stitching up at 4000 RPM by use of a cotton yarn number 50 for example.
Increased tension rendered to the upper thread necessitates the tension rendered to the lower thread to be increased because a knot formed by the upper and lower threads must be positioned in the center of thickness of a material at the time of stitch tightening. This increased tension rendered to the upper thread causes puckerings and material shrinkings to a sewn product, thus resulting in inferior quality of the sewn product.
In consequence, in the conventional sewing machine, when a low tension is rendered to the upper thread to form lock stitch seams, puckerings and material shrinkings are not caused to the sewn product. However, when seams are formed at high speed, lock stitches are not formed in the material, but, loops of the upper thread remain at the undersurface of the material, thus presenting the disadvantage that proper lock stitches cannot be formed.
In contrast thereto, when a high tension is rendered to the upper thread to form lock stitches, the lock stitches are formed in the material, however, puckerings and material shrinkings are caused to the sewn product, thus presenting the disadvantage that the market value of the sewn product is lowered.